The present invention relates to an exposure control device for a camera for determining the exposure value based on the brightness of an object to be photographed and the film sensitivity.
Conventionally, in an electronically controlled camera, there has been provided a device for controlling the determining the exposure value.
In such a device, the exposure control value Evs is determined based on the brightness Bv of an object to be photographed, the speed value Sv corresponding to the film sensitivity, and the compensation value .alpha. of the open aperture value (open f-No). If the exposure value Evs exceeds a threshold value Evsfl for the strobe flashing, which is the threshold value between the exposures with and without strobe flashing, the exposure control (shutter speed and aperture control) in the AE interlocking range is carried out without the strobe being flashed. If the exposure value Evs is lower than the threshold value for strobe flashing Evsfl, the strobe is flashed by the aperture value Av which is determined by an FM operation on the basis of the guide number of the strobe and the range finding data, and the shutter is controlled with the minimum exposure control value Evsmin.
In the AE (automatic exposure) interlocking range, the minimum exposure control value is fixedly determined in order to prevent influence due to camera-shake, and the deviation of the film sensitivity is utilized only for changing the minimum brightness of the AE interlocking range. Accordingly, if a film having a relatively high sensitivity is used, it will be possible to shoot a darker object.
The deterioration due to camera-shake will become remarkable when the magnification of the object increases, i.e., when the focal length of a zoom lens is increased. For this reason, in a conventional camera having a zoom lens, the minimum shutter speed has been controlled in such a fashion that the minimum shutter is delayed when the zoom lens is located at the WIDE side, and is accelerated at the TELE side when making automatic shooting.
Tables A and B show one of the examples of the threshold value for the strobe flashing and the lower limit of the AE interlocking range of a conventional camera. Table A shows the values at the WIDE side, and Table B shows the values at the TELE side. The Lv in the Tables is the value obtained by subtracting 5 from the object brightness Bv.
TABLE A ______________________________________ WIDE ISO 100 ISO 200 ISO 400 ______________________________________ Threshold Brightness 9.00 8.00 7.00 for Strobe Flashing (Lv) Threshold Exposure Value 9.00 9.00 9.00 for Flashing (Evsfl) Lower limit of 9.00 8.00 7.00 AE Interlock Range (Lv) Minimum Shutter Speed 9.00 9.00 9.00 Control Value (Evsmin) Shutter speed (sec.) 1/45 1/45 1/45 ______________________________________
TABLE B ______________________________________ TELE ISO 100 ISO 200 ISO 400 ______________________________________ Threshold Brightness 10.75 9.75 8.75 for Strobe Flashing (Lv) Threshold Exposure Value 9.50 9.50 9.50 for Flashing (Evsfl) Lower limit of 10.75 9.75 8.75 AE Interlock Range (Lv) Minimum Shutter Speed 9.50 9.50 9.50 Control Value (Evsmin) Shutter speed (sec.) 1/64 1/64 1/64 ______________________________________
In the conventional system, however, as the minimum shutter speed control value Evsmin and the threshold value Evsfl for strobe flashing are fixed according to the each of the focal length, influence due to the camera-shake remains unchanged even though film having high sensitivity is used.